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+---
+name: webui-dev
+description: Build interactive WebUI example apps with FAST-HTML hydration, template syntax, and component patterns.
+---
+
+# WebUI App Development with FAST-HTML
+
+Use this skill when building or modifying example apps under `examples/app/`.
+
+WebUI server-renders HTML at request time. FAST-HTML hydrates the pre-rendered DOM on the client, making it interactive without a full re-render.
+
+## Project structure
+
+Every example app follows this layout:
+
+```
+examples/app/<name>/
+├── src/
+│   ├── index.html              # HTML shell + CSS design tokens
+│   ├── index.ts                # Hydration bootstrap
+│   └── <component-name>/      # One directory per component
+│       ├── <component-name>.ts
+│       ├── <component-name>.html
+│       └── <component-name>.css
+├── data/
+│   └── state.json              # Initial state for SSR
+├── package.json
+└── tsconfig.json
+```
+
+### package.json
+
+```json
+{
+  "type": "module",
+  "scripts": {
+    "start:client": "esbuild src/index.ts --bundle --outfile=dist/index.js --format=esm --sourcemap --watch",
+    "start:server": "cargo run -p webui-cli -- start ./src --state ./data/state.json --plugin=fast --servedir ./dist --port 3001 --watch",
+    "start": "cargo xtask dev <name>"
+  },
+  "devDependencies": {
+    "@microsoft/fast-element": "catalog:",
+    "@microsoft/fast-html": "catalog:",
+    "esbuild": "catalog:",
+    "tslib": "catalog:",
+    "typescript": "catalog:"
+  }
+}
+```
+
+Use `catalog:` for all dependency versions — they resolve from `pnpm-workspace.yaml`.
+
+### tsconfig.json
+
+Required settings for FAST-HTML:
+
+```json
+{
+  "compilerOptions": {
+    "experimentalDecorators": true,
+    "useDefineForClassFields": false
+  }
+}
+```
+
+`useDefineForClassFields: false` is **mandatory** — FAST decorators rely on legacy class field behavior.
+
+## Template syntax (HTML)
+
+### Signal interpolation — `{{}}`
+
+Binds server-side state values into HTML. Processed by the WebUI parser at build time.
+
+```html
+<!-- Text content -->
+<p>{{userName}}</p>
+
+<!-- Attributes -->
+<a href="{{url}}">{{linkText}}</a>
+
+<!-- Nested properties -->
+<span>{{user.profile.name}}</span>
+
+<!-- Inside loops (item properties) -->
+<for each="item in items">
+  <div>{{item.title}}</div>
+</for>
+
+<!-- Inline styles -->
+<div style="background-color: {{color}}">...</div>
+```
+
+### Raw (unescaped) interpolation — `{{{}}}`
+
+Passes HTML through without entity encoding:
+
+```html
+<div>{{{rawHtmlContent}}}</div>
+```
+
+### Loop rendering — `<for>`
+
+```html
+<for each="item in items">
+  <my-component
+    name="{{item.name}}"
+    value="{{item.value}}"
+  ></my-component>
+</for>
+```
+
+- Loop variable is scoped to the `<for>` block and its children.
+- Nested loops can access outer loop items via their moniker.
+- Array length: `{{items.length}}`. Array indexing: `{{items.0.name}}`.
+
+### Conditional rendering — `<if>`
+
+```html
+<if condition="isVisible">
+  <div>Shown when truthy</div>
+</if>
+
+<if condition="count > 10">
+  <div>Comparisons work</div>
+</if>
+
+<if condition="isActive && role == 'admin'">
+  <div>Logical operators work</div>
+</if>
+```
+
+Supported: `==`, `!=`, `>`, `<`, `>=`, `<=`, `&&`, `||`, `!`. Max 5 operators per expression.
+
+### Boolean attributes — `?`
+
+Rendered only when the bound value is truthy:
+
+```html
+<input ?disabled={{isDisabled}} />
+<div ?hidden={{!isVisible}}>...</div>
+```
+
+### Complex (pass-through) attributes — `:`
+
+For structured data that passes through as-is:
+
+```html
+<my-component :config={{settings}}></my-component>
+```
+
+## FAST-HTML component patterns
+
+### Component class
+
+Every component extends `RenderableFASTElement(FASTElement)`:
+
+```typescript
+import { FASTElement, attr, observable } from '@microsoft/fast-element';
+import { RenderableFASTElement } from '@microsoft/fast-html';
+
+export class MyComponent extends RenderableFASTElement(FASTElement) {
+  @attr name!: string;              // HTML attribute (string, reflected)
+  @observable items!: ItemData[];   // Reactive property (triggers re-render)
+
+  inputRef!: HTMLInputElement;      // Template ref target (via f-ref)
+
+  async prepare(): Promise<void> {
+    // Hydration hook — read state from pre-rendered DOM
+  }
+
+  connectedCallback(): void {
+    super.connectedCallback();
+    // Post-hydration setup (event listeners, focus, etc.)
+  }
+}
+
+MyComponent.defineAsync({
+  name: 'my-component',
+  templateOptions: 'defer-and-hydrate',
+});
+```
+
+### `@attr` — HTML attributes
+
+- Reflected to/from the HTML attribute on the element.
+- **Use `!:` (no initializer)** for fields set in `prepare()`. Class field initializers run AFTER `super()`, overwriting values set during hydration.
+- Hyphenated HTML attributes map via `@attr({ attribute: 'display-value' })`.
+
+### `@observable` — Reactive properties
+
+- Triggers FAST template re-rendering when changed.
+- Use for data that drives `<for>` loops or conditional display.
+- **Use `!:` (no initializer)** for fields set in `prepare()`.
+
+### `prepare()` — Hydration hook
+
+Called during hydration to initialize component state from the pre-rendered DOM. This is **the** place to read server-rendered data:
+
+```typescript
+async prepare(): Promise<void> {
+  // Read @attr values from HTML attributes (initializers haven't run yet)
+  this.mode = this.getAttribute('mode') || 'default';
+
+  // Read child elements rendered by SSR
+  const items: ItemData[] = [];
+  for (const el of this.shadowRoot!.querySelectorAll('my-item')) {
+    items.push({
+      id: el.getAttribute('id') || '',
+      title: el.getAttribute('title') || '',
+    });
+  }
+  this.items = items;
+}
+```
+
+Rules:
+
+- Always read `@attr` values from `this.getAttribute()` — decorators initialize AFTER `prepare()`.
+- Read child element data from `this.shadowRoot.querySelectorAll()`.
+- Guard with `if (!this.shadowRoot) return;` when appropriate.
+- Avoid `<for>` loops over `@observable` string arrays — use object arrays or static HTML.
+
+### Component template (HTML)
+
+The `<template shadowrootmode="open">` wrapper is **optional** — the framework adds it automatically when absent. Only include it when you need to attach event listeners or other directives to the template root:
+
+```html
+<!-- Minimal — no root-level bindings needed -->
+<div>{{title}}</div>
+<for each="item in items">
+  <child-component name="{{item.name}}"></child-component>
+</for>
+
+<!-- With root-level event listeners — wrapper required -->
+<template shadowrootmode="open"
+  @custom-event="{onCustomEvent(e)}"
+>
+  <div>{{title}}</div>
+</template>
+```
+
+### Component styles (CSS)
+
+Scoped to the component via shadow DOM:
+
+```css
+:host {
+  display: block;
+}
+
+:host([state="active"]) .indicator {
+  background: var(--color-accent);
+}
+
+.content {
+  padding: var(--spacing-m);
+}
+```
+
+Use `:host([attr="value"])` selectors to style based on attribute state.
+
+## Event handling
+
+### Template event binding — `@event`
+
+Bind DOM and custom events to component methods in the template:
+
+```html
+<!-- Standard DOM events -->
+<button @click="{onClick()}">Click</button>
+<input @keydown="{onKeydown(e)}" />
+
+<!-- Custom events (bubble up from children) -->
+<template shadowrootmode="open"
+  @toggle-item="{onToggleItem(e)}"
+  @delete-item="{onDeleteItem(e)}"
+>
+```
+
+Pass `e` to receive the event object. Omit it when not needed.
+
+### Emitting custom events — `$emit`
+
+Child components emit events to parent components:
+
+```typescript
+// Emit a custom event with detail data
+this.$emit('toggle-item', { id: this.id });
+
+// Parent catches it via @toggle-item="{onToggleItem(e)}" on its <template>
+```
+
+Events bubble through the shadow DOM boundary via `composed: true`.
+
+### Template refs — `f-ref`
+
+Bind a DOM element to a component property for direct access:
+
+```html
+<input f-ref="{myInput}" @keydown="{onKeydown(e)}" />
+```
+
+```typescript
+myInput!: HTMLInputElement;  // Populated after hydration
+
+onSubmit(): void {
+  const value = this.myInput.value;
+  this.myInput.value = '';
+  this.myInput.focus();
+}
+```
+
+## Hydration bootstrap (index.ts)
+
+The entry point registers components and activates hydration:
+
+```typescript
+performance.mark('app-hydration-started');
+
+import { TemplateElement } from '@microsoft/fast-html';
+
+// Side-effect imports register components
+import './my-app/my-app.js';
+import './my-item/my-item.js';
+
+TemplateElement.options({
+  'my-app': { observerMap: 'all' },
+  'my-item': { observerMap: 'all' },
+}).config({
+  hydrationComplete() {
+    performance.measure('app-hydration-completed', 'app-hydration-started');
+    console.log('Hydration complete!');
+  },
+}).define({
+  name: 'f-template',
+});
+```
+
+- List **every** component in `.options()` with `{ observerMap: 'all' }`.
+- `.define({ name: 'f-template' })` triggers hydration.
+- Performance marks measure hydration time.
+
+## State (data/state.json)
+
+Provides initial values for SSR template rendering:
+
+```json
+{
+  "textdirection": "ltr",
+  "language": "en",
+  "title": "My App",
+  "items": [
+    { "id": "1", "name": "First", "status": "active" }
+  ]
+}
+```
+
+Top-level keys become template signals (`{{title}}`). Arrays drive `<for>` loops. The state is passed to the WebUI CLI via `--state`.
+
+## CSS design tokens
+
+Define design tokens as CSS custom properties in `index.html`:
+
+```html
+<style>
+  :root {
+    --color-brand-primary: #0078d4;
+    --spacing-m: 12px;
+    --border-radius-m: 6px;
+    --font-family-base: 'Segoe UI', sans-serif;
+  }
+  * { box-sizing: border-box; margin: 0; padding: 0; }
+  body { font-family: var(--font-family-base); }
+</style>
+```
+
+Components reference tokens via `var(--token-name)`. WebUI hoists `var()` usages at build time into the protocol for host-language token resolution.
+
+## Dev workflow
+
+```bash
+# Run dev server (builds + serves + watches)
+cargo xtask dev <app-name>
+
+# Or run client and server separately:
+pnpm start:client    # esbuild watch
+pnpm start:server    # webui-cli dev server
+
+# Production build
+webui build ./src --out ./dist --plugin=fast
+```
+
+The `cargo xtask dev` command auto-discovers apps from `examples/app/` — no registration needed.